The present invention relates, in general, to a tensioner, and more particularly to a tensioner applied for a chain-type driving member, and being of a type having a piston guided slidably in a bore of a housing and spring-loaded against the driving member, and a damping device for damping movements of the piston by a discharge of hydraulic fluid such as engine oil from a pressure chamber via at least one leak gap of the damping device.
A chain tensioner of this type is disclosed, for example, in German Pat. No. 40 15 708 and includes an annular disk arranged in the cavity of a hollow cylindrical piston which is closed at one end by a bottom. The annular disk has one flat end face which bears against the piston bottom whose inner wall includes radially extending throttling channels which open radially outwards into an annular throttling channel. The annular disk is provided with a through-bore which communicates with these throttling channels. The pressure chamber extends into the interior of the hollow piston. The engine oil which is pressurized by inward movements of the piston flows through the through-bore of the annular disk into the throttling channels and ultimately via an opening in the piston bottom to the outside of the piston i.e., out of the tensioner.
In tensioners of this type, the damping device should be matched very closely to the operational conditions of the tensioner, whereby particular importance is being given to the configuration of the leak gap and the throttling channels because the cross section of flow provided in the throttling channels has a major influence on the damping of the piston. In the case of the referred-to conventional tensioner, the throttling channels are arranged in a crossing pattern which complicates the production. If the throttling channels are made in an extrusion process, excessive variations from specified dimensions can occur in certain cases. A higher precision in shape and size can certainly be obtained by making the throttling channels through a machining process but this involves more work and higher costs. If the piston is of single-piece structure i.e., if the piston skirt and bottom are made as an integral component, it is very difficult, and in the case of very long piston skirts, even impossible to make the leak and throttling channels on the inner wall of the piston bottom.